FIG. 1 is a diagram illustrating an example of a mode of utilizing the wireless relay device.
The wireless relay device relays radio waves by amplifying and thus transmitting the radio waves received from a wireless base station toward an area at a far distance from the wireless base station and an area with an extremely low intensity of receiving the radio waves such as within a building and in an underground shopping mall. In cellular mobile communications, as in FIG. 1, the wireless relay device, when relaying between the wireless base station and a terminal, performs bidirectional relay, i.e., a case of receiving the signal from the wireless base station and transmitting the signal to the terminal and a case of receiving the signal from the terminal and transmitting the signal to the wireless base station.
It is an important problem of how to prevent oscillations caused when the signal amplified and thus transmitted by the wireless relay device is received by the receiving antenna of a self-device and further amplified. A contrivance for preventing the oscillations involves taking large isolation between the transmitting antenna and the receiving antenna of the wireless relay device. To be specific, schemes are exemplified such as setting a far distance between the transmitting antenna and the receiving antenna of the wireless relay device, providing a shielding object between the transmitting antenna and the receiving antenna, and adjusting directivity and polarization of each of the transmitting antenna and the receiving antenna.
The radio waves, however, get wrapped around the receiving antenna from the transmitting antenna due to reflections, diffractions, transmissions, etc, depending on an ambient environment. Therefore, a general method involves employing a coupling loop interference cancellation technology of canceling interference waves getting wrapped around the receiving antenna from the transmitting antenna by signal processing in combination with the scheme of physically ensuring the isolation between the transmitting antenna and the receiving antenna. For example, if a relay gain of the wireless relay device is set to 70 dB, it is desirable to ensure attenuation of the coupling loop interference having 80 dB or above as a total value of the isolation between the transmitting antenna and the receiving antenna and coupling loop interference suppression performance in order to ensure 10 dB or above as an S/N ratio (Signal to Noise Ratio) of the signal to be relayed.
Thus, the isolation between the transmitting antenna and the receiving antenna and the coupling loop interference cancellation technology are important to the wireless relay device. Especially in the case of the large coupling loop interference cancellation performance, the restriction of the isolation between the transmitting antenna and the receiving antenna can be reduced. Hence, there is a decreased restraint of a method of installing the wireless relay device, and introduction of the wireless relay device can be facilitated.
A specific coupling loop interference cancellation method is exemplified by the following technologies (Non-Patent document 1, Patent document 1, Patent document 2). In a scheme of canceling the signal, obtained in a way that multiples the transmission signal by the coupling loop interference cancellation weight, from the reception signal, there is a method of calculating a coupling loop interference cancellation weight by an adaptive algorithm so as to minimize signal power after being cancelled.
FIG. 2 is a diagram illustrating an example of a configuration of the conventional wireless relay device.
The wireless relay device in FIG. 2 uses the coupling loop interference cancellation method based on the adaptive algorithm. A signal into which the signal from the wireless base station is synthesized with the coupling loop interference is inputted to a receiving antenna. A receiver executes processes such as extracting a necessary band with a filter, conducting a conversion into a baseband signal with a down-converter, adjusting a signal level under AGC (Auto Gain Control), and performing a conversion into a digital signal with an A/D (Analog to Digital) converter. In a transmitter, the digital signal is converted again into the analog baseband signal by a D/A (Digital to Analog) converter. The analog baseband signal is, after being converted into an RF (Radio Frequency) signal by an up-converter and amplified by an amplifier, transmitted from the transmitting antenna.
Inputted to a coupling loop interference cancellation weight generator are a signal after the coupling loop interference cancellation and a signal obtained by delaying the transmission signal corresponding to the coupling loop interference. The coupling loop interference cancellation weight generator generates a coupling loop interference cancellation weight W for minimizing the signal power after the coupling loop interference cancellation based on an LMS (Least Mean Square) algorithm. A value, which is acquired in a way that multiples the signal acquired by delaying the transmission signal by the coupling loop interference cancellation weight W, is subtracted from the reception signal. A subtracted result is a signal after the coupling loop interference cancellation. The signal after the coupling loop interference cancellation becomes a transmission signal.
In the technology described above, a pilot signal is transmitted from the wireless relay device, and the coupling loop interference cancellation weight is calculated by estimating a state of a coupling loop interference propagation path. The coupling loop interference cancellation method using the pilot signal enables a propagation characteristic of the coupling loop interference to be calculated directly, however, unnecessary interference waves other than the signals to be relayed are transmitted because of transmitting the pilot signals.    [Non-Patent document 1] Kazuto Itoh and one other, “Booster Using Adaptive Coupling loop interference Canceller”, Report of Technical and Research, compiled by the Institute of Electronics and Communication Engineers of Japan, August in 1999, RCS99-78.    [Patent document 1] Japanese Patent Laid-Open Publication No. 2003-174392    [Patent document 2] Japanese Patent Laid-Open Publication No. 2005-236626
A wireless relay device disclosed herein adopts the following means.